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The 42-year-old was arrested, along with a string of others, in December 2014 and was yesterday sentenced in the High Court in Whangarei to 14 years and four months in prison for his role as one of the four cooks involved in manufacturing meth.

The sentence followed his earlier guilty pleas to two charges of manufacturing methamphetamine and one of possession of equipment with an intention to manufacture methamphetamine.

Justice Simon Moore said it was puzzling that Lang, who he described as an intelligent and perceptive man, allowed himself to be drawn into an illegal activity after working as an engineer for 15 years.

At least 9kg of meth, with a street value of between $3.2 million and $4.5 million, was cooked over 10 weeks at a property in Taipuha Rd at Waiotira, between Whangarei and Paparoa, between September and December 2014.

Lang was involved in two cooking phases.

The other three cooks were Anthony Hone Mangu, Jaydean Hura and Elijah Rogers, who were sentenced earlier.

Mangu was jailed for 15 years, Hura 16 years and eight months, and Rogers 19 years.

Justice Moore said Lang moved into his mother's house, situated next door to the drug lab, before the drug operation began.

While socialising with his neighbours, who were later identified as principal offenders in the drug manufacturing, they learnt Lang was an engineer and encouraged him to set up sophisticated equipment for the illegal operation.

"But it was your decision. Nobody forced you and, regrettably, you agreed to become involved and that involvement became more embedded when you became addicted to methamphetamine," Justice Moore said.

"Your role was not confined to just a passing knowledge but you had scientific knowledge which ensured the methamphetamine operation was successful."

Crown prosecutor Richard Annandale said Lang was the "troubleshooter" for other cooks who identified problems during the drug operation, fixed them, and made suggestions to ensure manufacturing of meth would continue.

Defence lawyer John Watson said, while Lang had technical knowledge, he could not be described as one of the principal offenders.

Lang was ordered to serve half of his sentence or seven years and two months before he could be eligible for parole.

The ringleader of the drug operation, Brownie Harding, together with Evanda Harding, Kiata Sonny Pene and Casey Rewha, will be sentenced on September 1.